1. Field of the Invention
This invention relates to a constant pressure sample cylinder with spheroid or ellipsoid shaped mixing element. Other generally ellipsoid shaped mixing elements are also disclosed. These mixing elements are capable of mixing a sample in the sample chamber when the cylinder is manually inverted. The force of gravity urges the mixing element to move through the sample which creates vortices and general agitation of the sample. This turbulence causes stratified samples with different specific gravities to be mixed. Another feature of this invention is an improved piston for use in constant pressure sample cylinders. As is well known in the art, it is desirable to accumulate sample in a vessel at a relatively high pressure. The pressure of the vessel is relatively high for the express purpose of confining the sample, and in particular, to prevent the sample from changing phase from a liquid to a gas. The device is able to be filled with sample to a maximum design capacity for the device, typically several hundred cubic centimeters. Pressures as high as 2,000 psi are not extreme.
While the sample is being accumulated over a period of days or weeks, the sample typically will stratify. The sample tends to stratify because it is typically a heterogeneous mix of different hydrocarbons with different specific gravities. For example, production from a wet gas well may include the following: methane, propane, butane, isobutane, natural gasoline and/or ethane. Some of these hydrocarbons may liquefy at ambient conditions and are referred to in the industry as "light liquids".
When it is time to remove the sample from the constant sample pressure cylinder, it is first necessary to mix the sample. After mixing, a lab technician will then draw off a portion of the sample for analysis in a gas chromatograph or other suitable instrument. The Btu of the sample is then calculated based on the content analysis from the gas chromatograph. This disclosure features a spheroid or ellipsoid shaped mixing element. The mixing element is in the form of an untethered solid which churns the stored sample and thereby mixes it when the cylinder is manually inverted. Motion is imparted to the mixing element by gravity.
Other alternative embodiments of the mixing element include a shape comprising two circumferences joined by a radius at the perimeter. This alternative embodiment is not a true spheroid or ellipsoid; however it will function in approximately the same manner and will avoid damage to the interior walls of the cylinder.
The piston has a depression on its lower face to nest with the mixing element. This mixing element and piston configuration enables the piston to be initially forced by pressure at the time of charging to the extreme of its travel, capturing the ellipsoid mixing element and nesting against the head at the end of the storage cylinder. This design captures the mixing element. When the alternative embodiment mixing element is used, the depression in the lower face of the piston and the head is sized to conform to the shape of the alternative mixing element.
The mixing element is untethered in the sample chamber. However, the forces of gravity will bring the mixing element to the bottom of the chamber. Untethered mixing elements can be dangerous in that they are shaken violently at the time of mixing and such mixing elements may damage the finish on the interior wall of the surrounding cylinder. This spheroid or ellipsoid mixing element is designed with sufficiently rounded exterior surfaces so that they do not scratch or mar the interior surface finish of the cylinder when they come in contact during mixing. The piston slides along the cylinder, thereby requiring a high quality finish. The high quality finish must be preserved to avoid leakage around the piston. The ellipsoid shaped mixing element in the preferred embodiment is manufactured from Celcon M-90, Kel-F or other suitable nonmaring materials. Celcon M-90 is a thermoplastic manufactured by Celanese Chemical Company from acetal. Kel-F is a series of fluorocarbon products manufactured by 3M Company including polymers of chlorotrifluorethylene and certain copolymers that are characterized by high thermal stability, resistance to chemical corrosion, high dialectric strength, high impact, tensil and compressive strength.
2. Description of the Prior Art
U.S. Pat. No. 4,459,865, assigned to Welker Engineering Company, discloses a constant pressure cylinder with vortex mixer. This prior art device contains a tethered mixing element which is more expensive to manufacture and maintain than the untethered ellipsoid shaped mixing element of the present invention. In addition the present invention has an improved piston design over the aforementioned patent.
U.S. Pat. No. 4,409,850 assigned to Y-Z Industries, Inc. discloses a constant pressure cylinder with untethered spherical mixer. The spherical mixer disclosed in this prior art device has a diameter which is generally in excess of one half the diameter of the cylinder. The present invention has superior mixing capabilities because of the shape of the spheroid or ellipsoid mixing element over a round ball. Another relevant reference is U.S. Pat. No. 3,789,670 assigned to Cities Service Oil Company which discloses a round ball as a vortex mixer, said ball generally having a diameter at least about one fourth as large as the inside diameter of the cylinder. Again the present invention has superior mixing capabilities to this reference.
Some prior art pistons knows to applicant had a magnet assembly positioned at the very front of the piston which caused a hammering effect when said piston was forcibly driven against either end cap. After repeated hammering the magnet assembly cracked and fell apart which resulted in scoring on the interior diameter of the cylinder. The present invention provides a piston with a means for cushioning the magnet assembly which is superior to these prior art designs.
Another device of interest is U.S. Pat. No. 3,789,670 of Rosenwald. The same inventor is listed on additional U.S. Pat. Nos. 3,793,886 and 3,793,888. The earlier patent of McKinney U.S. Pat. No. 2,636,387 is also noted. An additional reference is U.S. Pat. No. 3,390,580 of Taylor. These references listed above are representative of devices over which the present application distinguishes.